The broad aim of this research is to evaluate whether mindfulness meditation training reduces loneliness, improves social relationship functioning, and improves biomarkers of inflammation in lonely older adults. Lonely older adults (aged 55+ years) are a population that is increasing in size in the United States relative to other segments of the population and they have significantly elevated health risks (e.g., increased inflammation, cardiovascular disease, depression, and premature mortality). Interventions that can reduce loneliness have significant potential to reduce the elevated morbidity and premature mortality in this growing but understudied patient population. Previous RCT studies that have attempted to reduce loneliness in older adults have had limited success. In a pilot study, we found that an 8-week Mindfulness-Based Stress Reduction (MBSR) program reduced loneliness and inflammation in lonely older adults (Creswell et al., 2012). This proposal describes an innovative and well-controlled RCT of MBSR in lonely older adults (N=188), comparing MBSR to a structurally equivalent 8-week Health Education Program (HEP) and a no treatment control condition. Participants will complete questionnaires, daily experience sampling (using pre-programmed smartphones), and provide blood samples at baseline, post-treatment, and 3-month follow-up. This proposed study will test three specific aims that are guided by a novel conceptual model: (1) mindfulness meditation training reduces loneliness; (2) the effects of mindfulness meditation training on loneliness are explained by reductions in relational distress in daily life interactions; and (3) mindfulness meditation training reduces biomarkers of inflammatory disease risk that are elevated in lonely older adults (pro-inflammatory gene expression, C Reactive Protein, and Interleukin-6). The information provided by this study has the potential to: (a) identify a novel treatment for loneliness in older adults; (b) reveal a social psychological mechanism for this effect (i.e., reductions in daily life relational distress); and to (c) describe the health protective consequences of this treatment on inflammatory disease risk in this at-risk patient population.